This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Recurrent difficulties with stability of the aging Varian Fieldial magnetic field regulation system presently employed for 16Ghz microimaging work has led us to explore interim methods for achieving adequate B0 stability. A successful method we have implemented on an interim basis for current ESR microimaging studies, pending installation of a new magnet power source and regulation system later this year, is a field/frequency-derived regulation patch based on an existing field/frequency lock (FFL) system. Using the FFL, we are able to maintain relative field stability on the order of 5ppm over the duration of a given ESR microscopy experiment, which is adequate for our immediate needs. An important aspect of this interim upgrade is that our experience gained in adding the current FFL patch yields helpful insight into the design requirements and practical solution for a hybrid (Hall/FFL) magnetic field regulation driver that will be installed with the new switchmode magnet power source in fall, 2008.